BALB/c mice fail to develop an antibody response after immunization with low doses of a polysaccharide antigen (PS) isolated from the extracellular, gram-negative bacterium Pseudomonas aeruginosa. These mice do generate a protective T cell mediated immune response. Development of an in vitro system in which T cells kill these bacteria has permitted a detailed analysis of the cellular basis for this killing. Studies using this in vitro system have demonstrated that T cells kill the bacteria directly through secretion of a bactericidal lymphokine. The in vitro assay has also be used to evaluate the T cell killing capability of different inbred mouse strains. It has been found that C.B20 mice, which differ from the BALB/c strain at only one genetic locus, are unable to generate T cell killing in vitro. Preliminary results indicate that the difference in in vitro T cell killing between BALB/c and C.B20 mice correlates directly with the ability to these two strains to survive in vivo bacterial challenge. The significance of the C.B20 T cell bacterial killing defect will be evaluated in two different models of P. aeruginosa infection, the burned mouse model and the leukopenic mouse model. The effect on survival of reconstituting the C.B20 T cell response with adoptive transfer of cells from BALB/c mice will also be studied. These studies should establish the in vivo significance of this T cell killing mechanism. A human in vitro assay of T cell mediated bacterial killing has also been developed. This assay will be used to characterize the cellular mechanisms involved in human T cell killing. In addition, we will analyze the adequacy of T cell killing in humans, such as cystic fibrosis patients, who, like the C.B20 mouse, appear to be particularly susceptible to P. aeruginosa infection. Establishing the importance of T cell immunity in the pathogenesis of P. aeruginosa infection represents an important first step in efforts to intervene immunologically to prevent this infection in the susceptible host.